In this fight, the boss will resummon the adds whenever any one of them is dead.
You do not need to clear adds to finish the battle, just the main boss would do.
To clear the OPTs efficiently, you need to use AoE abilities/SB such as Bladeblitz, summons, Sephiroth’s SB, etc. Note that OPTs are immune to earth (RIP Quake). Good thing is that the adds have low health and they have no status immunity.

You can control the fight by either paralysing or stopping the boss immediately after clearing the little adds. This will buy you a decent number of turns to push damage as much as you can while there’s only 1 target up.

Alternatively, since the boss doesn’t have high health at all, you can just ignore the adds and focus single target abilties on the main boss.

The Lost Number consists of two halves; the purple half which performs physical attacks and the red half which casts magic. After a certain threshold, the Lost Number will change completely into one of his sides.

He changes form depending on the attack that caused it to reach the threshold:

If the damage was inflicted by a magic attack, Lost Number will turn red and gain a substantial increase in MAG and RES. It will attack with powerful magic from then on. In this state, it has lower DEF.

If the damage was inflicted by a physical attack, Lost Number will turn purple and gain a boost in ATK and DEF. It will attack with strong physical attacks from then on. This form has low RES.

If the damage was inflicted via Poison ticks, no effect will happen until Lost Number takes a physical or magic damage. Thus, if the player chooses to defeat Lost Number with Poison only, he will never change form.

Intimidate and follow it with Armor/Mental Break(down) to take it down quick. You could also Poison it if you want for extra dps.

Generally it doesn’t really matter what form it changes into. If anything, just react accordingly. That is, if it turns purple, Power Break it; if it turns red, Magic Break it.

This dude has really high health. In addition, it has no elemental weakness.
Nevertheless, it can be paralysed, blinded and poisoned. Intimidate and Poison will be king here. Follow it with Armor/Mental Break(down) to boost overall dps.

If you foresee yourself not killing it before all your Intimidate charges expire, bring Power and Magic Break (the latter especially since Materia Keeper gains a new ability at critical HP, which is a huge party-wide lightning damage)

Don’t worry so much about losing medals to Action and/or Damage Taken, as long as you can keep Cloud and Sephiroth alive, you’ll most probably master this boss with ease.
(FYI, total of 45 medals in this dungeon, you can lose a maximum of 7 medals to master, a lot of leeway!)

UPDATE: If you observe that Intimidate is only lasting a turn on Materia Keeper, it is because it has insanely high MND, which reduces the duration of debuffs by a considerable amount. Consider using other longer lasting debuff like Poison and Blind instead.

Those of us who have Cloud’s MC should remember how this fight goes. The only difference from the story dungeon is that this one has higher health.

Slow is incredibly useful here, and for such an attrition battle, Sentinel Grimoire is definitely the top pick for friend summon, unless you have one of your own, which in that case, Lunatic High is recommended.

You can also inflict Jenova with Berserk and she will only use Tail Laser and nothing else.

If Jenova BIRTH is berserked, she will use her Brsk/Conf Ability with normal Cast Time, but then immediately follow it up with her usual ‘15% chance of Stop/85% chance of double attack’ as interrupts. The double attack is picked randomly, though is still affected by Berserk’s targetting rules. If Jenova BIRTH’s last random attack was Stop, then her Brsk/Conf ability will also be Stop for the next turn only.

Quoting /u/tfmurphy from reddit with regards to Jenova’s AI when Berserked.

Muketsu provided two sample parties to take on Jenova:

[Sample team comp #1] (Non Retaliate):
(assume a decent amount of 5* gear and abilities are honed to at least r4)

Sephiroth: Power Break / Armor Break

Cloud: Leg Shot / Drain Strike

Tidus: Leg Shot / Power Break

Tyro: Protectga / Drain Strike

Garnet: Curaga / Haste

[Sample team comp #2] (Retaliate):
(assume main hitter has 5* weapon with RS and abilities are honed to at least r4)

Sephiroth: Retaliate / Power Break

Cloud: Double Cut / Double Cut

Tyro: Double Cut / Armor Break

Garnet: Slowga / Protectga

Aerith: Curaga / Haste

Recommended RW: Sentinel Grimoire, Lunatic High, Advance

EX: Calamity from The Skies

[Total required stamina: 72]

Mastery Reward

Major Wind Orb x1, Major Growth Egg x1

Completion reward

Gil × 30000

First Completion Reward

Power Breakdown, Greater Summon Orb x3

BOSS 1

Sample:H0512/ Sample:H0512-OPT

Stamina 18, Stages 1, experience value 3000

Special score conditions

(i) Defeat Sample:H0512 without being KO’d.

HP

► Sample:H0512: ~38000
► Sample:H0512-OPT: ~7000

Commentary

Nothing is changed here, just focus fire the main boss with single target abilities.

Boss 2

Lost Number

Stamina 18, Stages 1, experience value 5000

Special score conditions

(i) Defeat Lost Number without being KO’d

HP

~72000

Commentary

Start with Slowga/Intimidate, and use this battle to charge up SB bars as much as you can.

2 Comments

A trick, especially for EX: To save mythril, put your characters (especially those that need skills most, like Black mage with high honed spells)just before lvl up threshold. Experience eggs can help that.
The characters will level up. I think I’ll try to get lvl up just before the last boss – it seems very hard, I’ve just managed to kill it on Elite with an effort…